Those players often then declare that Ghazghkull is leaving the unit to trudge ahead. Let's say for instance that he moves 4". They then take the boy unit he was with and moves them up '6 and re-attaches Ghaz to them. This is not legal....

Of course there are other ways to do it, I'm just saying the comments just shows that more people do not approve of it. The article itself have not given in depth details on that issue, like not even stating why it's not legal.

One particular comment suggests you can only intend to either join or leave the same unit, and that intention carries on for the rest of the movement phase, indicating you can't change that intention at the end of the phase.